During the course of the Thieves Guild questline, Karliah reveals that Mercer was part of the "Nightingale Trinity", and is guilty of betraying Nocturnal by taking the Skeleton Key from the Twilight Sepulcher. Mercer then used the key to commit crimes against the Guild until his activities were discovered by his predecessor, Gallus, whom he then murdered.

After you have successfully completed Taking Care of Business, Brynjolf will lead you into the Cistern to speak with the Thieves Guild guildmaster, Mercer Frey. Brynjolf will then initiate conversation with him:

Brynjolf:"Mercer? This is the one I was talking about... our new recruit."Mercer:"This better not be another waste of the Guild's resources, Brynjolf."

Mercer will then turn to you: "Before we continue, I want to make one thing perfectly clear. If you play by the rules, you walk away rich. You break the rules and you lose your share. No debates, no discussions... you do what we say, when we say. Do I make myself clear?" You can now choose to respond to him in three different ways:

"This better not be another waste of the Guild's resources, Brynjolf."

Your Dialogue

Mercer's Dialogue

"Yes, I understand."

"Good. Then I think it's time we put your expertise to the test."

"I'm not sure."

"If you're not sure maybe you don't belong here. I'll ask again, are we clear on all of this?"

"Rules? We're thieves. What's the point of rules?"

"I'll let that comment go because you're new here. Ask things out of turn again, and we have a problem. Now, are we clear on all of this?"

Once you agree with what Mercer has said, Brynjolf will interrupt the conversation:

Brynjolf:"Wait a moment, you're not talking about Goldenglow, are you? Even our little Vex couldn't get in."Mercer:"You claim this recruit possesses an aptitude for our line of work. If so, let him/her prove it. Goldenglow Estate is critically important to one of our largest clients. However, the owner has suddenly decided to take matters into his own hands and shut us out. He needs to be taught a lesson. Brynjolf will provide you with the details."Brynjolf:"Mercer, aren't you forgetting something?"Mercer:"Hmm? Oh, yes. Since Brynjolf assures me you'll be nothing but a benefit to us, then you're in. Welcome to the Thieves Guild."

You are now officially a member of the Thieves Guild and now the next quest, Loud and Clear will begin. If you approach Mercer at any other time, he will generally say to you: "This better be important, I'm quite busy." or "Don't you have better things to do than disturb me?"

After you have completed the quest Dampened Spirits you are to speak to Mercer. He will then begin the conversation by saying: "Ah, there you are. I've consulted my contacts regarding the information you recovered from Goldenglow Estate, but no one can identify that symbol." Exclaiming that the symbol found on the Promissory Note from Honningbrew Meadery is the same as the one found on the Goldenglow Bill of Sale from the Goldenglow Estate, he will remark: "It would seem our adversary is attempting to take us apart indirectly by angering Maven Black-Briar. Very clever." From this point, you can respond in three different ways:

Your Dialogue

Mercer's Dialogue

"You admire them?"

"They're well-funded and they've been able to avoid identification for years. I'm impressed it reached this point."

"Maybe we should recruit them."

"You jest, but they've been able to avoid identification for years. They're obviously well-funded, driven and patient."

"Clever or not, they need to die."

"Don't dismiss our adversary so easily. They're well-funded, patient and have been able to avoid identification for years."

"No, it.. it can't be. I haven't heard that name in decades."

Mercer will then continue to say: "Just don't mistake my admiration for complacency; our nemesis is going to pay dearly." Asking how the adversary is going to pay for it, he will explain: "Because, even after all their posturing and planning, they've made a mistake. The parchment you recovered mentions a "Gajul-Lei." According to my sources, that's an old alias used by one of our contacts. His real name is Gulum-Ei. Slimy bastard." Also asking where you should begin, he will tell you: "Gulum-Ei is our inside man at the East Empire Company in Solitude. I'm betting he acted as a go-between for the sale of Goldenglow Estate and that he can finger our buyer. Get out there, shake him down and see what you come up with. Talk to Brynjolf before you leave if you have any questions." You are now directed to go speak with Gulum-Ei in Solitude.

When you return to the Guild with information on the buyer, Mercer will ask you: "Did Gulum-Ei give up any information on our buyer?" If you killed Gulum-Ei after gaining the information, Mercer's approach will be slightly different: "I thought it was painfully obvious that Gulum-Ei was important to the Guild. I'm sorry to say your payment for the job will have to go to elsewhere since you feel like taking things in your own direction. Now, did Gulum-Ei give up any information on our buyer?" Telling him that the buyer was someone named Karliah, he will say: "No, it.. it can't be. I haven't heard that name in decades. This is grave news indeed, she's someone I hoped to never cross paths with again." Stating that Gulum-Ei also mentioned that she was a murderer, Mercer will go on to explain: "Karliah destroyed everything this Guild stood for. She murdered my predecessor in cold blood and betrayed the Guild. After we discovered what she'd done, we spent months trying to track her down, but she just vanished." If you ask him why she has returned, he will state: "Karliah and I were like partners. I went with her on every heist. We watched each other's backs. I know her techniques, her skills. If she kills me, there'll be no one left that could possibly catch her. If only we knew where she was..." Also telling him that Gulum-Ei told you that she said "Where the end began.", he will reply with: "There's only one place that could be. The place where she murdered Gallus... a ruin called Snow Veil Sanctum. We have to go out there before she disappears again." Asking if you're going along with him, he will answer: "Yes, I'm going with you and together we're going to kill her. Prepare yourself and meet me at the ruins as soon as you can. We can't let her slip through our fingers." You are now directed to meet Mercer outside of Snow Veil Sanctum and the quest Speaking With Silence will begin.

"Let's get moving, I want to catch her inside while she's distracted."

As you arrive at Snow Veil Sanctum, you will find Mercer waiting for you outside the entrance. Approaching him, he will initiate the conversation: "Good, you're finally here. I've scouted the ruins and I'm certain Karliah is still inside." Asking if he saw Karliah, he will correct you: "No, I found her horse. Don't worry, I've taken care of it... she won't be using it to escape. Let's get moving, I want to catch her inside while she's distracted. Take the lead." You can choose to respond to him in either of two ways:

Your Dialogue

Mercer's Dialogue

"You want me to lead?"

"I'm sorry, I was under the impression I was in charge. You're leading and I'm following. Does that seem clear to you?"

"Understood."

"Just make certain you keep your eyes open. Karliah is as sharp as a blade. The last thing I need is you blundering into a trap and warning her that we're here."

"Quite simple really, I don't know what the fuss is about these locks."

Regardless of what you chose to respond with, you will have to inevitably agree with Mercer's decision. He will then become a 'temporary' follower and will follow you to the locked door leading into the Snow Veil Catacombs. As you wait for Mercer, he will approach the door and begin a running commentary while unlocking the door: "They say that these ancient Nordic burial mounds are sometimes impenetrable. This one doesn't look too difficult. Quite simple really, I don't know what the fuss is about these locks. All it takes is a bit of know-how and a lot of skill." After Mercer has managed to unlock the door, he will say: "That should do it. After you." Upon entering the ruins, Mercer will comment about the surroundings: "The stench in here... this place smells of death. Be on your guard."

"Fortunately, these doors have a weakness if you know how to exploit it. Quite simple, really."

As you progress through the ruins, Mercer will often warn you about several traps along the way. The first trap you will come across is a spike trap, and Mercer will warn you: "Pull the chain over there, and watch out for the spikes. Looks like Karliah reset all of the traps." Another trap further along is a tripwire which activates a fire trap, where Mercer will warn you again by saying: "There's two more of them ahead." Proceeding onward, you will come across a room with several bone chimes placed around the room. Mercer will warn: "Bone chimes... clever. Rigged to wake the draugr I'd bet. Don't blunder into any of them." As you make your way through into the sanctum, Mercer will be quick to comment: "We're on the right track. She's been through here as well." Before you continue, you will come across a wooden double-door which leads onto a large chamber. Mercer will advise you about the door ahead as you approach it: "That door up ahead... looks perfect for hiding an ambush. Be ready."

As you move forward you will come across a Nordic puzzle door, which only Mercer can unlock. As he is unlocking the door he will comment: "Ah, it's one of the infamous Nordic puzzle doors. How quaint. Without the matching claw, they're normally impossible to open. And since I'm certain Karliah already did away with it, we're on our own. Fortunately, these doors have a weakness if you know how to exploit it. Quite simple, really. Karliah's close. I'm certain of it. Now let's get moving." Once you step through into the next chamber, Karliah will shoot you with a poisoned arrow, which will cause you to fall to the ground immobilized. Mercer will then approach Karliah, and a conversation between the two can be seen:

"Come, Karliah. It's time for you and Gallus to become reunited!"

Mercer:"Did you honestly think your arrow will reach me before my blade finds your heart?"Karliah:"Give me a reason to try."Mercer:"You're a clever girl, Karliah. Buying Goldenglow Estate and funding Honningbrew Meadery was inspired."Karliah:""To ensure an enemy's defeat, you must first undermine his allies." It was the first lesson Gallus taught us."Mercer:"You always were a quick study."Karliah:"Not quick enough, otherwise Gallus would still be alive."Mercer:"Gallus had his wealth and he had you. All he had to do was look the other way."Karliah:"Did you forget the Oath we took as Nightingales? Did you expect him to simply ignore your methods?"Mercer:"Enough of this mindless banter! Come, Karliah. It's time for you and Gallus to become reunited!"Karliah:"I'm no fool, Mercer. Crossing blades with you would be a death sentence. But I can promise you the next time we meet, it will be your undoing."

Karliah will then disappear using an invisibility potion. Mercer will approach you as you lie on the ground, and say: "How interesting. It appears Gallus's history has repeated itself. Karliah has provided me with the means to be rid of you, and this ancient tomb becomes your final resting place. But do you know what intrigues me the most? The fact that this was all possible because of you. Farewell. I'll be certain to give Brynjolf your regards." He will then stab you and walk away, leaving you to die within the sanctum.

Once you have made your way through the Dwarven ruins of Irkngthand to the Irkngthand Sanctuary, you will see Mercer Frey trying to pry the Eyes of the Falmer from the statue. A conversation will then occur:

Karliah:"He's here and he hasn't seen us yet. Brynjolf, watch the door."Brynjolf:"Aye, lass. Nothing's getting by me."Karliah:"Climb down that ledge and see if you can..."Mercer:"Karliah, when will you learn you can't get the drop on me?"

At that point, the ledge will break off and Mercer will engage in conversation with you: "When Brynjolf brought you before me I could feel a sudden shift in the wind. And at that moment, I knew it would end with one of us at the end of a blade." You now have two options with which to reply back:

Your Dialogue

Mercer's Dialogue

"Give me the Key, Mercer."

"What's Karliah been filling your head with? Tales of thieves with honor? Oaths rife with falsehoods and broken promises? Nocturnal doesn't care about you, the Key or anything having to do with the Guild."

"If anyone falls, it will be you."

"Then the die is cast, and once again my blade will taste Nightingale blood!"

If you choose the second option, the conversation will end and you will engage in combat with Mercer. However, if you chose the first option, you are able to continue the conversation for a while longer, as shown below:

Your Dialogue

Mercer's Dialogue

Dialogue Option 1

"I don't believe you. Nocturnal guides me."

"Then it appears the shadows shroud more than your presence... they blind your wisdom as well. Our actions have always been one in the same; both of us lie, cheat and steal to further our own end."

"The difference is I still have honor."

"It's clear you'll never see the Skeleton Key as I do... as an instrument of limitless wealth. Instead you've chosen to fall over your own foolish code."

"If anyone falls, it will be you."

"Then the die is cast, and once again my blade will taste Nightingale blood!"

Dialogue Option 2

"It's not about Nocturnal. This is personal."

"Revenge is it? Have you learned nothing from your time with us? When will you open your eyes and realize how little my actions differ from yours? Both of us lie, cheat and steal to further our own end."

"The difference is I still have honor."

"It's clear you'll never see the Skeleton Key as I do... as an instrument of limitless wealth. Instead you've chosen to fall over your own foolish code."

"If anyone falls, it will be you."

"Then the die is cast, and once again my blade will taste Nightingale blood!"

Dialogue Option 3

"If anyone falls, it will be you."

"Then the die is cast, and once again my blade will taste Nightingale blood!"

Dialogue Option 4

"To blazes with Nocturnal. I'm here for the Eyes."

"Wait a moment... do I detect a hint of genuine avarice from this noble thief? Perhaps Karliah and Brynjolf misjudged you and your true nature is no different than my own."

"The difference is I still have honor."

"It's clear you'll never see the Skeleton Key as I do... as an instrument of limitless wealth. Instead you've chosen to fall over your own foolish code."

"If anyone falls, it will be you."

"Then the die is cast, and once again my blade will taste Nightingale blood!"

Once the conversation comes to an end, Mercer will initiate combat with you, and will taunt Karliah: "Karliah, I'll deal with you after I rid myself of your irksome companions. In the meantime, perhaps you and Brynjolf should get better acquainted." Brynjolf will then be controlled to fight off Karliah, as a direct result of Mercer using his Agent of Subterfuge power. At multiple times during the fight, he will turn invisible. When Mercer is killed, you can loot the Skeleton Key and the Eyes of the Falmer (needed for The Litany of Larceny) from his body.

A map can be found in Mercer's basement leading to his whereabouts, as well as the unique sword Chillrend.

If you start the Speaking With Silence quest and meet Mercer in front of Snow Veil Sanctum, you can then fast travel away and he will follow you as a permanent member of your party. He is skilled with one-handed weapons and is unkillable. However, he cannot be reverse pickpocketed, so he is only useful for his sword, and he greatly hinders your ability to sneak.

The voice for Mercer is provided by Stephen Russell, who has also voiced Garrett, a Master Thief and the protagonist of the Thief series of games.

Mercer's two weapons, a leveled dagger and Dwarven sword, can be pickpocketed early on, leaving him weaponless in the final battle for the Skeleton Key.